As energy costs rise and the cost of producing LEDs fall, LED lighting systems are increasingly looked to as a viable alternative to more conventional systems, such as those employing incandescent, fluorescent, and/or metal-halide bulbs. One long-felt drawback of LEDs as a practical lighting means has been the difficulty of obtaining white light from an LED. Two mechanisms have been supplied to cope with this difficulty. First, multiple monochromatic LEDs were used in combinations (such as red, green, and blue) to generate light having an overall white appearance. More recently, a single LED (typically blue) has been coated with a phosphor that emits light when activated, or “fired” by the underlying LED (also known as phosphor-conversion (PC) LEDs). This innovation has been relatively successful in achieving white light with characteristics similar to more conventional lighting, and has widely replaced the use of monochromatic LED combinations in LED lighting applications. Monochromatic LED color combinations are commonly used in video, display or signaling applications (light to look at), but almost never used to illuminate an area (light to see by). As even a relatively dim light can be seen, the luminous intensity generated by LEDs in video or display applications is not a major concern.
PC LEDs, however, are highly expensive to produce relative to more conventional bulbs (as are LEDs, generally) and efficiency and longevity gains of PC LEDs (PC LEDs produce light less efficiently than monochromatic LEDs due to the two-step process required to generate the white light) were not perceived to offset the high initial costs, except in applications where efficiency and longevity were more highly valued. Such applications include lighting systems powered by limited-capacity power sources, such as batteries, and particularly systems with batteries charged by “off-grid” energy sources such as photovoltaic (“PV”) panels, wind turbines, and small hydro-turbines. Even when LEDs (particularly, PC LEDs) were used in a LED lighting system, the practice (until the present invention) has been to use as few LEDs as necessary to achieve the desired luminance by operating each LED at its maximum current capacity.
In connection with the increasing use of LEDs for certain lighting applications, two methods of allowing a user to control the intensity of LEDs have been developed (though in many applications, such a simple LED flashlight, no intensity adjustment can be made by the user). The first, simply varying the forward current (like most diodes, LEDs only allow current to pass in one direction) passing through the LED, has largely been used only in applications where efficiency and/or precise selection of a range of luminous intensities is not a concern (e.g., in an automotive brake light where only two intensity levels are desired and the automobile's alternator generates far more electricity than is required to power the LED brake light). Typically, a voltage divider circuit with one or more variable resistors is used to vary the voltage drop across the LED, which in turn results in a proportionally varied current. Such a method of controlling luminous intensity is inefficient because the power dissipated in the resistor is simply lost, thus reducing the overall efficiency, particularly when lower currents are being supplied. However, the costs of these relatively simple circuits can be significantly less than the constant-current drivers discussed below.
In applications where more precise intensity control is desired (e.g., many, though not necessarily all, lighting system applications), or greater efficiency is required (e.g., systems for use with a limited-capacity power source, such as a PV panel and/or battery) a constant-current driver (CCD) is used to supply a substantially constant current to the LED, regardless of the supplied voltage. It is possible to supply a substantially constant current using “passive” components (e.g., resistors and capacitors, and the like), though these passive means do not necessarily yield efficiency increases over simpler voltage divider circuits because power losses are still associated with the passive components. The more efficient constant current control is typically achieved by “active” switching, in which actively controlled components (e.g., internal, gated, bi-polar transistors (IGBTs), and the like) are used to supply the substantially constant current without the losses associated with passive components.
In constant current systems, the luminous intensity of the LED is varied, typically, by using a pulse-width modulated (PWM) control signal to vary the duty cycle with which the CCD supplies the substantially constant current to the LED. When the PWM control signal has a frequency of over approximately 100 Hz, the cycling of the LED is not visually perceivable. For example, a PWM control signal with a frequency of 1000 Hz will turn the LED ON and OFF 1000 times per second. If 50% intensity is desired, the PWM control signal will provide for ON and OFF periods of equal duration. For 75% intensity, the ON periods will be three times longer than the OFF periods. For 25% intensity, the OFF periods will be three times longer than the ON periods. No flashing or occulting will be perceivable to the human eye because of the high frequency. Instead, the eye will perceive a constant, but diminished, intensity as the duty cycle is decreased from 100% intensity. (Intensity, as used herein, refers to luminous intensity, and may be perceived and/or actual, unless otherwise specified.) In conventional PWM lighting, selecting the maximum intensity (no OFF periods) will result in all LEDs operating at a maximum rated current.
To maximize the power available from a limited-capacity power source, such as a PV panel and battery system, charge controllers for batteries have been employed using a technique known as Maximum Power Point Tracking (“MPPT”). MPPT maximizes the charge rate when power generation conditions are sub-optimal (e.g., for a PV panel, a day with relatively few day-light hours). MPPT charge controllers are very expensive and have previously been used only in relatively high current systems (with charging currents over 20 amps) and not in connections with limited-capacity power sources used to power lighting systems (in which the charging current is typically less than 10 amps), as the efficiency gains in lower current systems were considered to be proportionally lower, and would not offset the added cost of a MPPT charge controller.